Method for forming a battery cell

Abstract

In a first aspect, the present disclosure relates to a method of forming a battery cell, comprising: - a carbon-containing anode, a sodium-containing cathode, and an electrolyte solution comprising an organic solvent containing an alkali metal bis(oxalato)borate in which the alkali metal ion is selected from sodium (Na + ) and potassium (K + ), and pyrrolidone and / or a phosphate ester compound; - performing a first formation cycle by fully charging and discharging the battery cell; - the formation cycle being performed at a temperature in the range above 25 °C to 100 °C. The present disclosure further relates to a method of forming a battery cell, comprising: - performing a first step of charging the battery cell at a C-rate within the range of 0.5C to 20C; - performing a second step of constant voltage (CV) charging at a voltage exceeding the decomposition potential of the electrolyte salt for a period until a C-rate current range within the range of 0.02C to 4C is reached.

Description

Technical Field

[0001] The present disclosure generally relates to a method of forming a battery cell. More particularly, the present disclosure relates to a method of forming a battery cell including a carbon-containing anode, a sodium-containing cathode, and an electrolyte solution including an alkali metal bis(oxalato)borate.

Background Art

[0002] Lithium-ion batteries dominate the rechargeable battery market. However, this technology has the drawback that lithium resources are relatively scarce. Although superior to previous-generation secondary battery technologies, lithium-ion batteries are not environmentally friendly and are costly from a recycling perspective.

[0003] Sodium-ion batteries are an attractive alternative to lithium-ion batteries and are perhaps the most viable means of supporting renewable energy sources for load leveling and excess energy storage purposes.

[0004] In the development of electrode materials having high charge storage capacity and rate capability, substantial research progress has been made, and the success of sodium-ion batteries is hampered by the unstable formation of the solid electrolyte interface (SEI).

[0005] The SEI is a layer formed on the anode surface in all alkali metal ion batteries that utilize a liquid electrolyte.

[0006] In a sodium-ion battery, sodium ions move from the cathode to the anode through the electrolyte during charging and then from the anode to the cathode during discharging. As the battery is cycled, a solid layer forms at the interface between the electrolyte and the electrode. This is the SEI layer, which is mainly formed from the decomposition products of the electrolyte. The SEI layer should ideally function as a passivation layer to prevent further decomposition of the electrolyte.

[0007] Therefore, SEI is an essential component for improving cycle stability in sodium-ion batteries. However, since the formation of SEI also leads to irreversible loss of capacity, it is important to understand the mechanism and optimize the SEI formation process.

[0008] Therefore, the present disclosure aims to improve the cycling conditions for optimizing the SEI formation process. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] In view of the above-mentioned and other drawbacks of the prior art, an object of the present disclosure is to provide an improvement regarding cycling conditions in a battery cell including a carbon-containing anode and a sodium-containing cathode. MEANS FOR SOLVING THE PROBLEMS

[0010] This object and other objects of the present disclosure can be achieved by a method of forming a battery cell according to claim 1 or claim 3. Further embodiments are described in the dependent claims, the following description, and the drawings.

[0011] According to a first aspect, the present disclosure relates to a method of forming a battery cell including a carbon-containing anode, a sodium-containing cathode, and an electrolyte solution including an alkali metal bis(oxalato)borate in which the alkali metal ion is selected from sodium (Na + ) and potassium (K + ) and an organic solvent including a pyrrolidone and / or a phosphate ester compound. The method includes - performing a first formation cycle by fully charging the battery cell and then discharging it in a subsequent step, wherein the formation cycle is performed at a temperature in the range of above 25°C to 100°C, including the step.

[0012] The SEI protects the anode by suppressing the movement of electrons from the anode to the electrolyte while allowing the movement of sodium ions from the electrolyte to the anode, making it very important for the battery. Therefore, a stable SEI is extremely important for achieving good cycle performance. Therefore, an ideal SEI is either an ion conductor or an electrical insulator.

[0013] In the method of forming a battery cell according to the present disclosure, in the battery cell according to the present disclosure, a first formation cycle is performed by fully charging the battery cell and then discharging it in a subsequent step. When the formation cycle is performed at a temperature in the range above 25°C to 100°C, surprisingly high discharge capacity and reduction of polarization can be achieved, which has been found by the inventors to be related to improved SEI formation with higher stability and smaller SEI. The electrolyte system according to the present disclosure is a low-cost, fluorine-free alternative to conventional electrolyte systems while having good cycle performance, such as efficient formation of a passive layer.

[0014] The first formation cycle may be performed at a temperature in the range of 30°C to 100°C, optionally in the range of 30°C to 80°C. It has been surprisingly found that this provides a consistent long-term high discharge capacity over multiple cycles.

[0015] According to a second aspect, the present disclosure provides a method of forming a battery cell comprising a carbon-containing anode, a sodium-containing cathode, and an electrolyte comprising an alkali metal bis(oxalato)borate in which the alkali metal ion is selected from sodium (Na + ) and potassium (K + ) and an organic solvent containing pyrrolidone and / or a phosphate ester compound, the method comprising: - a first step of charging the battery cell at a C-rate in the range of 0.5C to 20C; - a second step of performing constant voltage (CV) charging at a voltage exceeding the decomposition potential of the electrolyte salt for a period until a C-rate current range of 0.02C to 4C is reached. The method is related to the above steps.

[0016] In the method of forming a battery cell according to the second aspect, in the battery cell according to the present disclosure, a first step of charging the battery cell within a range of 0.5C to 20C of the C-rate, that is, the rate at which the battery is charged (or discharged); and a second step of performing constant voltage (CV) charging at a voltage exceeding the decomposition potential of the electrolyte salt for a period until a C-rate current range of 0.02C to 4C is reached. The inventors have surprisingly found that higher discharge capacity and higher initial Coulombic efficiency are obtained. It has been found that the surprisingly high discharge capacity and initial Coulombic efficiency are related to the improved SEI formation provided by the method. If the formed SEI is insufficient, this will rather lead to continuous electrolyte degradation, and thus lower discharge capacity, lower initial Coulombic efficiency, and higher polarization.

[0017] The decomposition potential of the electrolyte salt of the electrolyte according to the present disclosure can be found in the paper "A Wide-Temperature-Range, Low-Cost, Flourine-Free Battery Electrolyte Based On Sodium Bis(Oxalate)Borate", Chem. Mater. 2021, 33, 4, 1130 - 1139; https: / / pubs.acs.org / doi / full / 10.1021 / acs.chemmater.0c03570#.

[0018] The C-rate is a measure of the rate at which a battery cell is charged or discharged, and in this context, it is related to the rate of charging the battery cell. The C-rate is defined as the current flowing through the battery cell divided by the theoretical current amount at which the battery outputs its nominal rated capacity in one hour.

[0019] In this specification, constant voltage (CV) charging means adjusting the current so as to reach and maintain a uniform voltage set for the battery cell.

[0020] The decomposition potential of an electrolyte salt refers to the minimum voltage between the anode and cathode of a battery cell required for electrolysis to occur.

[0021] In the method according to the second aspect, the second step may be performed until a C-rate current range of 0.05C to 1C is reached. As a result, it has been found that the discharge capacity becomes higher and the initial Coulomb efficiency increases.

[0022] In the method according to the second aspect, the C-rate in the first step may be in the range of 0.5C to 15C. It has been found that this is beneficial for cycle performance in terms of discharge capacity and Coulomb efficiency.

[0023] In the method of forming a battery cell according to the first and / or second aspect, the concentration of the salt in the electrolyte solution may be in the range of 0.3M to 1.5M, preferably in the range of 0.3M to 0.8M. The inventors have confirmed that such a concentration of the salt in the electrolyte solution in the battery cell according to the present invention leads to good cycle performance for forming a stable passive layer.

[0024] In the method of forming a battery cell according to the first and / or second aspect, the cathode material may be a sodium iron(II) hexacyanoferrate(II) material.

[0025] In the method of forming a battery cell according to the first and / or second aspect, the anode material may be hard carbon.

[0026] In the method of forming a battery cell according to the first and / or second aspect, the sodium iron(II) hexacyanoferrate(II) material may have the formula Na 2-y Fe[Fe(CN)6]·mH2O (where y < 0.2 and 0 < m < 2).

[0027] In the method of forming a battery cell according to the first and / or second aspect, the organic solvent has the following structure:

Chemical formula

[0028] In the method of forming a battery cell according to the first and / or second aspect, the organic solvent may be N-methyl-2-pyrrolidone (NMP). Such a solvent has an improved ability to dissolve electrolyte salts.

[0029] In the method of forming a battery cell according to the first and / or second aspect, the organic solvent has the following structure:

Chemical formula

[0030] In the method of forming a battery cell according to the first and / or second aspect, the organic solvent may be trimethyl phosphate (TMP). Such a solvent has an improved ability to dissolve electrolyte salts.

[0031] In the method of forming a battery cell according to the first and / or second aspect, the organic solvent of the electrolyte solution may be triethyl phosphate (TEP), trimethyl phosphate (TMP), N-methyl-pyrrolidone (NMP), or a mixture thereof. Such a solvent has an improved ability to dissolve electrolyte salts.

[0032] In the method of forming a battery cell according to the first and / or second aspect, the organic solvent of the electrolyte solution may contain a mixture of N-methyl-2-pyrrolidone (NMP) and trimethyl phosphate.

[0033] In the method for forming a battery cell according to the first and / or second aspect, the organic solvent may contain N-methyl-2-pyrrolidone (NMP) and trimethyl phosphate (TMP) in the range of 40:60 to 90:10. Such a solvent has an advantageous ability to dissolve electrolyte salts.

[0034] In the method for forming a battery cell according to the first and / or second aspect, the concentration of the salt in the electrolyte solution may be 0.3 to 1.5 M, preferably 0.4 to 0.8 M, more preferably 0.5 to 0.6 M. This provides a high-performance electrolyte solution that is fluoride-free and low-cost.

[0035] In the method for forming a battery cell according to the first and / or second aspect, the conductivity of the electrolyte solution may exceed 4 mS / cm at a temperature of 23°C.

[0036] Further features and advantages of the present invention will become apparent upon examination of the appended claims and the following description. Those skilled in the art will understand that, without departing from the scope of the present invention, different features of the present invention can be combined to create embodiments other than those described below.

[0037] Some exemplary embodiments will be described with reference to the accompanying drawings.

Brief Description of the Drawings

[0038]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0039] Hereinafter, the present disclosure will be described more fully with reference to the accompanying drawings in which exemplary embodiments are shown. However, the present disclosure can be implemented in many different forms and should not be construed as limited to the embodiments described herein. Rather, these embodiments are provided for thoroughness and completeness. The same reference numerals refer to the same elements throughout the specification.

[0040] FIG. 1 shows the schematic principle of a sodium battery 1 including a carbon-containing anode 2, i.e., a negative electrode, and a sodium-containing cathode 3, i.e., a positive electrode. The sodium-containing cathode 3 is a sodium source that utilizes sodium ions 4 as charge carriers. The sodium battery 1 stores energy in the chemical bonds of the anode 2. When the battery 1 is charged, Na + ions 3 are forcibly deintercalated from the cathode 3 and moved toward the anode 2. During discharge, the process is reversed. When the circuit is completed, electrons return from the carbon-containing anode 2 to the sodium-containing cathode 3, and Na + ions 3 return to the cathode 3. As shown in FIG. 1, during discharge of the battery cell 1, oxidation occurs at the anode 2, while reduction occurs at the cathode 3. The direction of current flow is determined by the battery cell voltage, which is the potential difference between the cathode 3 and the anode 2.

[0041] The cathode and the anode are such that the alkali metal ions are sodium (Na + ) and potassium (K +It is separated by an electrolyte solution 5 containing an alkali metal bis(oxalato)borate selected from ), and an organic solvent containing pyrrolidone and / or a phosphate ester compound. Preferably, the concentration of the salt in the electrolyte solution is in the range of 0.3 M to 1.5 M, preferably in the range of 0.3 M to 0.8 M. Preferably, the organic solvent has the following structure:

Chemical formula

[0042] As used herein, the term "battery" means a device including one or more electrochemical cells.

[0043] In an "electrochemical cell", chemical energy is converted into electricity by reduction and oxidation (redox) reactions at the electrodes.

[0044] As used herein, "cycle" refers to a constant current galvanostatic cycle unless otherwise specified.

[0045] The cathode material is preferably a sodium hexacyanoferrate(II) iron(II) material, for example, a sodium hexacyanoferrate(II) iron(II) material having the formula Na 2-y Fe[Fe(CN)6]·mH2O (where y < 0.2 and 0 < m < 2).

[0046] The anode is a carbon-containing anode, preferably a hard carbon-containing anode.

[0047] The conductivity paired with the good rate capability of the cathode material enables high-speed charging and discharging.

[0048] The battery cell 1 may further include a separator 6 for preventing an electrical short circuit between the anode and the cathode and providing mechanical stability to the battery cell. The separator material can be a polymer film generally made from any chemically stable and electrically insulating material, such as polypropylene, polyethylene, or a combination thereof.

[0049] Figure 2 shows the hard carbon particles 7 from the anode material 2 (shown in Figure 1) and the electrolyte solution 5, with an SEI layer 8 formed on the anode surface 2a. As the battery 1 is cycled, a solid layer is formed at the interface between the electrolyte solution 5 and the anode material.

[0050] Figure 3 schematically shows a method for forming the battery cell 1 shown in Figure 1 according to a first aspect of the present disclosure. The battery cell includes a carbon-containing anode, a sodium-containing cathode, and an electrolyte solution containing an alkali metal ion selected from sodium (Na + ) and potassium (K + ) and an organic solvent containing an alkali metal bis(oxalato)borate and a pyrrolidone and / or a phosphate ester compound. In a first step, a first formation cycle is performed by fully charging the battery cell, and in subsequent steps, the battery cell is discharged. The formation cycle is performed at a temperature in the range above 25°C to 100°C. Further cycle steps may be performed at room temperature, i.e., a temperature of 20 - 25°C.

[0051] Figure 4 schematically shows a method for forming the battery cell 1 shown in Figure 1 according to a second aspect of the present disclosure. The battery cell includes a carbon-containing anode, a sodium-containing cathode, and an alkali metal ion selected from sodium (Na + ) and potassium (K +An electrolyte solution comprising an alkali metal bis(oxalato)borate selected from ), a pyrrolidone, and / or an organic solvent containing a phosphate ester compound. The method includes a first step of charging a battery cell at a C-rate within the range of 0.5C to 20C, and a second step of performing constant voltage (CV) charging at a voltage exceeding the decomposition potential of the electrolyte salt for a period until reaching a C-rate current range of 0.02C to 4C.

[0052] Figure 5 shows the measurement results of the discharge capacity of a battery cell formed according to the first aspect of the present disclosure. The formation cycles were performed at temperatures of 30°C, 50°C, and 70°C according to the present disclosure, and the comparative measurements were performed when the formation was carried out at room temperature. Three battery cells were formed at each temperature, and after a total of 12 battery cells were formed, galvanostatic cycles were performed at C / 5 at room temperature in cycles 2 to 5. The measurements were performed with a cycle number of up to 5 cycles.

[0053] The measurements show the discharge capacity obtained from the first cycle formation at a C-rate of C / 5 at room temperature, 30°C, 50°C, and 70°C.

[0054] As can be seen from Figure 5, the discharge capacity is high over multiple cycles at each temperature, especially when the formation cycles are performed at 50°C and 70°C.

[0055] Figure 6 shows a comparison between the discharge capacity of a battery cell according to the present disclosure that was subjected to a formation cycle according to the second aspect (Protocol 2) and the discharge capacity of a similar battery cell that was not subjected to a formation cycle (without Protocol 2). Subsequently, galvanostatic cycles were performed at C / 3 in subsequent cycles for all battery cells. As shown in Figure 6, the discharge capacity of the battery cell subjected to the formation cycle according to the present disclosure was maintained high over time and was significantly higher than the discharge capacity of the comparative battery cell.

[0056] FIG. 7 is a bar graph showing the Coulomb efficiency measured after forming a battery cell according to the second aspect (protocol 2) of the present disclosure, compared to the same type of battery cell subjected to a constant current galvanostatic cycle (without protocol 2), i.e., the same cycle conditions as cycles 2 to 75. The measurements were made after the first cycle, the second cycle, and up to the 75th cycle. The average value of the Coulomb efficiency was measured for cycles 3 to 75. As can be seen from FIG. 7, when the battery cell is formed according to the second aspect, the Coulomb efficiency is significantly improved in the first cycle.

Claims

1. A carbon-containing anode, a sodium-containing cathode, and an electrolyte solution containing an organic solvent containing an alkali metal bis(oxalato)borate in which the alkali metal ion is selected from sodium (Na ＋ ), potassium (K ＋ ), and a pyrrolidone and / or a phosphate ester compound, the method for forming a battery cell including: - A step of performing a first formation cycle by fully charging and discharging a battery cell, - A step in which the formation cycle is performed at a temperature within a range exceeding 25°C to 100°C, A method for forming the above battery cell, comprising:

2. The method for forming a battery cell according to claim 1, wherein the first formation cycle is performed at a temperature within a range of 30°C to 100°C, for example, 30°C to 80°C, preferably 45°C to 80°C.

3. A carbon-containing anode, a sodium-containing cathode, and an electrolyte solution containing an organic solvent containing an alkali metal bis(oxalato)borate in which the alkali metal ion is selected from sodium (Na ＋ ), potassium (K ＋ ), and a pyrrolidone and / or a phosphate ester compound, the method for forming a battery cell including: - A step of performing a first step of charging the battery cell at a C-rate within a range of 0.5C to 20C, - A step of performing a second step of constant voltage (CV) charging at a voltage exceeding the decomposition potential of the electrolyte salt for a period until a C-rate current range of 0.02C to 4C is reached, The above method, comprising:

4. The method according to claim 3, wherein the second step is performed until a C-rate current range of 0.05C to 1C is reached.

5. The method according to claim 3 or 4, wherein the C-rate in the first step is within a range of 0.5C to 15C.

6. A method for forming a battery cell according to any one of claims 1 to 5, wherein the concentration of the salt in the electrolyte solution is within a range of 0.3M to 1.5M, preferably within a range of 0.3M to 0.8M.

7. A method for forming a battery cell according to any one of claims 1 to 6, wherein the cathode material is a sodium iron (II) hexacyanoferrate (II) material.

8. A method for forming a battery cell according to any one of claims 1 to 7, wherein the anode material is hard carbon.

9. A method of forming a battery cell according to any one of claims 1 to 8, wherein the sodium iron (II) hexacyanoferrate (II) material has the formula Na ２－ｙ Fe[Fe(CN) ６ ·mH ２ O (where y < 0.2 and 0 < m < 2).

10. The organic solvent has the following structure: 【Chemical 1】 (wherein R is selected from alkyl groups containing 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms) is N-alkyl-2-pyrrolidone, and a method for forming a battery cell according to any one of claims 1 to 9.

11. A method for forming a battery cell according to any one of claims 1 to 10, wherein the organic solvent is N-methyl-2-pyrrolidone (NMP).

12. The organic solvent has the following structure: [Chemical Formula 2] (wherein R １ , R ２ , and R ３ are independently selected from alkyl groups containing 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms), the method for forming a battery cell according to any one of claims 1 to 8.

13. A method according to any one of claims 1 to 8 or claim 12, wherein the organic solvent is trimethyl phosphate (TMP).

14. A method according to any one of claims 1 to 8, wherein the organic solvent of the electrolyte solution contains a mixture of N-methyl-2-pyrrolidone (NMP) and trimethyl phosphate.

15. The method according to claim 14, wherein the organic solvent contains N-methyl-2-pyrrolidone (NMP) and trimethyl phosphate (TMP) in the range of 40:60 to 90:

10.

16. The method according to any one of claims 1 to 15, wherein the concentration of the salt in the electrolyte solution is 0.3 to 1.5 M, preferably 0.4 to 0.8 M, more preferably 0.5 to 0.6 M.

17. The method according to any one of claims 1 to 16, wherein the conductivity of the electrolyte solution exceeds 4 mS / cm at a temperature of 23 °C.

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